R.W.H. Butler1, S. Spencer2 & H.M. Griffiths1
1 Department of Earth Sciences, The University of Leeds, Leeds LS2
9JT, United Kingdom
2 Department of Geology, American University of Beirut, Beirut, Lebanon.
Abstract: Structural evolution along continental transform faults may be related to fault zone geometry and to regional variations in plate kinematics. Using a case study of the Lebanese sector of the Dead Sea Transform, the finite geometry of transpression at a restraining bend can be shown to have evolved in time. Relative structural chronologies, calibrated against dated landscape features such as lava-covered palaeosurfaces, coastal erosion surfaces and their incised drainage basins, are used to establish the timing of displacement activity on the major transcurrent faults. The early part of the region's structural history, up until late Miocene times, was controlled by the geometry of the through-going Yammouneh Fault. Transpression on this right-trending left-lateral structure was accommodated by strike-slip and distributed crustal shortening represented by the initial uplift of Mount Lebanon. For the past 6 Myr the principal active strand of the transform has been the Roum Fault. For much of this period it is presumed to have been a through-going fault which accommodated about 30 km left-lateral displacement. During the Quaternary, the fault zone has become strongly segmented. Although the location of active transcurrent faulting has migrated during the history of the transform, the major site of crustal shortening, the Mount Lebanon - Jabel Barouk structure, has remained broadly fixed. However the rates of amplification of this structure and the coastal flexure appear to have varied. Ongoing uplift of Mount Lebanon and local Plio-Quaternary folding suggest that the offshore continuation of the Roum Fault contains a rightwards, transpressive bend. We relate this multistage history for the Lebanese sector of the transform to an evolving plate tectonic setting: the rotation pole for relative plate motion between Africa and Arabia has migrated through time and the triple junction between the transform and the Tethyan destructive plate margin to the north has moved from onshore SE Turkey to now lie in the NE Mediterranean. Our case study illustrates the transient and evolving nature of deformation in continental restraining bends.
Complete reference: Butler, R.W.H., Spencer, S. & Griffiths, H. 1998. Structural evolution of a transform restraining bend - transpression on the Lebanese sector of the Dead Sea Fault System. In: Continental transpressional and transtensional tectonics (ed. R.E. Holdsworth , R.A. Strachan & J.F. Dewey) Spec. Publs. Geol. Soc. London 135, 81-106